Metallic pigments are known to provide electrochemical, electrical, thermal, and other properties to compositions which are used for protecting various materials such as metal from corrosion, maintaining electrical conductivity, shielding equipment from electromagnetic fields, resisting elevated temperatures, and providing protection from moisture. Silver, gold and other noble metal pigments are used for their electrical conductivity and thermal conductivity properties. Zinc and magnesium are used for their electrochemical properties. Aluminum is used for its thermal and chemical barrier properties. A major shortcoming of the noble metals is their strong cathodic potential. When used in products for electrical and thermal management, the noble metals coupled with anodic materials such as aluminum alloys are used for electrical equipment.
For example, metals such as zinc and magnesium are used in cured coatings to provide corrosion resistance to the metal on which they are coated. Typical zinc-rich primers use zinc “dust” which is approximately 5 micron zinc powder. This zinc powder is added untreated to various resins, organic and inorganic. Zinc-rich coatings are used mostly on steel to slow down the onset of rust or corrosion. A common secondary problem with zinc-rich coatings is the rusting or corrosion of the zinc powder in the coating while it is protecting the steel. When zinc corrodes, it typically forms a white residue which can discolor the object being protected and is not desired for aesthetic reasons. This zinc self-corrosion also “uses up” the zinc and reduces the effective life of the zinc-rich coating.
Other metals, such as magnesium has been used in combination with zinc and by itself in similar coatings to protect steel and aluminum respectively. Magnesium also is prone to forming white corrosion products which discolor the object being protected and is undesirable for aesthetic reasons. A second application of coatings with metal pigments is for electrical and thermal conductivity. Silver, nickel, copper and aluminum are good conductors of electricity and heat. Silver and nickel are commonly used as pigments in conductive coatings on other materials like glass, carbon/graphite, and aluminum which are lighter and less expensive. Copper is an excellent bulk conductor but is not typically used as a conductive pigment as it oxidizes quickly and loses its ability to conduct electricity effectively in coatings. Aluminum is an excellent bulk conductor, but it also oxidizes easily in the natural environment and is not effective as a conductive pigment in coatings. A third application is the protection of iron and iron alloy (steel) particles from rusting due to exposure to the environment. These particles are used in coatings for their magnetic properties and tend to red rust and lose effectiveness over time due to exposure to the environment.
Specifically, this invention relates to a composition and to a process for preparing and applying a semi-conducting coating comprising at least one metal oxide onto metal particles and the use of these coated particles in coatings designed to protect substrates from corroding and provide electrical or thermal conductivity.
More specifically, this invention relates to compositions of these coated particles in various compositions, such as greases and other vehicles which are used to protect substrates from corroding (zinc or magnesium) and to provide an electrically and thermally conductive path on surfaces which have insufficient conductivity (silver, nickel, aluminum, copper) or for magnetic properties (iron). The conductive coatings are typically used for electromagnetic shielding, static dissipation, continuity, and thermally conductive pathways in the case of flexible circuits and similar applications. These metals are typically used at high purity for maximum conductivity or coating efficiency. This invention covers all of these potential alloys as long as the key property of cathodic protection (magnesium and zinc) or electrical conductivity (silver, nickel, copper and aluminum) are maintained. For example, zinc can be alloyed with nickel to yield a particle with tailored open-circuit potential and controlled activity. This alloy can be coated effectively with the semi-conducting coating described by this invention to control the corrosion or white rusting of the zinc in the alloy.
It is therefore an object of this invention to incorporate electrochemically active coated-pigments into a binder to provide cathodic protection to metal substrates.
It is another object to provide cathodic protection to metal substrates by coating the substrate with a sacrificial-anode coating that keeps the electrochemical potential of the substrates negative to prevent its corrosion.
It is a further object of this invention to provide metal pigments containing effective amounts of a corrosion-resistant oxide coating and the use of these oxide coated pigments in film-forming binders as a coating for metal substrates.